Waxy feedstocks may be used to prepare basestocks having a high VI. However, in order to obtain a basestock having low temperature properties suitable for the intended use, it is usually necessary to dewax the waxy feedstock. Dewaxing may be accomplished by solvent means or catalytic means. Solvent dewaxing is a physical process whereby waxes are removed by contacting with a solvent such as methylethylketone followed by chilling to crystallize the wax and filtration to remove the wax. Catalytic dewaxing involves converting the hydrocarbons leading to unfavorable low temperature properties to hydrocarbons having more favorable low temperature properties. Long chain normal paraffins and slightly branched paraffins readily solidify and thus result in generally unfavorable low temperature properties. Catalytic dewaxing is the process of converting these long chain normal paraffins and slightly branched paraffins to molecules having improved low temperature properties.
Catalytic dewaxing may be accomplished using catalysts that primarily dewax by cracking waxes to lower boiling products, or by catalysts that primarily isomerize waxes to more highly branched products. Catalysts that dewax by cracking decrease the yield of lubricating oils while increasing the yield of lower boiling distillates. Catalysts that isomerize do not normally result in significant boiling point conversion.
Catalytic dewaxing may be accomplished selectively using zeolites that primarily function as cracking zeolites and having a pore size and shape that admits n-paraffins and slightly branched paraffins but excludes more highly branched molecules, cycloaliphatics, and aromatics. Such zeolites are exemplified by ZSM-5, ZSM-11, ZSM-12, and offretite.
Catalytic dewaxing may also be accomplished by use of zeolites that perform selective branching on n-paraffins. These hydroisomerizing zeolites convert n-paraffins to iso-paraffins that have more favorable low temperature properties. Examples of such isomerizing zeolites include ZSM-22, ZSM-23, SSZ-32, ZSM-35, and ZSM-48. The amount of isomerization vs. cracking is typically controlled by process conditions.
There is still a need for improved dewaxing processes that achieve good yields of high VI lube base oils having good low temperature properties.